Water is central to this play, and, particularly, the act of being immersed in water – namely, drowning. Of course, the first scene when the ship splits is a pretty good time to worry about drowning, but the imagery goes beyond that to represent loss and recovery.
When first exiled with Miranda, Prospero suggests that he could have drowned the sea with his own tears when he cried over his lost dukedom and his past: "When I have deck'd the sea with drops full salt" (1.2.18).
The new inhabitants of the isle are obsessed with water too. Ferdinand, upon hearing Ariel's song, knows it refers to his father's certain drowning:
Full fathom five thy father lies;
Of his bones are coral made;
Those are pearls that were his eyes;
Nothing of him that doth fade
But doth suffer a sea-change
Into something rich and strange (1.2.20)
Ariel's song leads Ferdinand to believe that his father has drowned and is lost to him forever. Not only that, but the song suggests that his body has been transformed into something unrecognizable.
Later, when Alonso gives up hope that Ferdinand could have survived the shipwreck he says, "he is drown'd / Whom thus we stray to find, and the sea mocks / Our frustrate search on land. Well, let him go" (3.1.1) and Antonio notes the King has "given up hope" (3.1.1).
The idea here is that when someone is lost to the sea, there probably isn't even going to even be a body that can be recovered. Drowning demands that the dead must be let go, without the closure of a burial ceremony. So what we're talking about here is the seeming finality of drowning. Usually, once a thing is given to the ocean, it can never be taken back, which is why Rose drops the diamond necklace into the ocean in everyone's favorite tacky love story (yes, Titanic).
But wait! The Tempest isn't just a story about loss. It's also about the recovery of what seems to have been lost forever. As we know, Ferdinand and his father don't actually drown and when they discover each other at the play's end, we're reminded that new beginnings are possible.
Same goes for Prospero, who once thought Milan would never be restored to him but lives to see the day his daughter is married to Prince Ferdinand and will live as a royal in Italy. While Miranda and Prospero will never get back the twelve years they lost on the island, the play suggests that, despite their suffering, they will gain something even greater.